1. Field of the Invention
The present invention relates to a mixed flow turbine and a mixed flow turbine rotor blade.
2. Description of the Related Art
As a machine which converts combustion gas energy into mechanical rotation energy efficiently, a radial turbine is known. FIG. 1A is a horizontal cross sectional view of a rotor blade 103 of the radial turbine, and FIG. 1B is a vertical cross sectional view of a rotor blade unit 100 of the radial turbine.
As shown in FIG. 1B, the radial turbine is provided with the rotor blade unit 100 attached to a rotation axis and a scroll 102 having a shape similar to a snail. The rotor blade unit 100 has a hub 101 and a plurality of blades 103 arranged on the hub 101 in a radial direction. A nozzle 104 is interposed between the scroll 102 and a rotating region of the blades 103.
A gas flows from the scroll 102 into the nozzle 104, and is accelerated and given rotation force by the nozzle 104 to produce high velocity flow 105, which flows into the direction of the rotor axis. The flow energy of the high velocity flow 105 is converted into the rotation energy by the blades 103 arranged on the hub 101. The blades 103 exhaust the gas 107 having lost the energy into the direction of the rotation axis.
As shown in FIG. 1A, the cross section of the blade 103 has a shape in which the blade 103 extends approximately linearly in the rotation axis direction in the neighborhood of a gas inlet from the surface of the hub, and then bends in a direction orthogonal to the rotation axis. Thus, the blade 103 is formed to be twisted smoothly into a direction orthogonal to the rotation direction from the hub side to the exhaustion side. Also, an upper edge of the blade 103 on the side of the nozzle 104 is flat and parallel to the rotation axis.
FIG. 2 shows a relation between the blade profile of the blade 103 in the view from the rotation axial direction and its inlet velocity triangle of the radial turbine. As shown in FIG. 2, U represents the rotation velocity of the blade 103 in the gas inlet, C represents an absolute flow velocity, and W represents a relative flow velocity W. The turbine efficiency is expressed in relation to a theoretical velocity ratio (=U/C0). Here, C0 shows the maximum flow velocity of the accelerated gas as fluid under the condition of given turbine inlet temperature and given pressure ratio. As shown in FIG. 3, the turbine efficiency η is maximized when the theoretical velocity ratio is around 0.7, and decreases parabolically in the region that the theoretical velocity U/C0 is larger than 0.7 and in the region that the theoretical velocity U/C0 is smaller than 0.7. As shown in FIG. 2, the velocity triangle is represented by U, C1 and W1 in the neighboring region of the maximum efficiency point A. The gas which flows into the radial turbine has a relative flow velocity W1 in a direction opposite to the radial direction, i.e., toward the center in the neighboring region A of the maximum efficiency point, and the incidence is approximately zero.
When this kind of turbine is used for a turbo charger, by increasing the fuel supplied to the engine for accelerating, the turbine inlet temperature rises. Also, the absolute flow velocity at the nozzle outlet increases as shown by C2 in FIG. 2, and the relative flow velocity W2 becomes diagonal to the blade 103. As a result, a non-zero incidence i2 is caused. The theoretical velocity C0 rises with the rise of the turbine inlet temperature, and the theoretical velocity ratio U/C0 decreases to the B point. Also, the turbine efficiency η decreases from the maximum efficiency point A to a lower efficiency point B with the generation of the incidence i2, as shown in FIG. 3. By increasing the supply of fuel, although one expects the rise of the number of the rotation, the turbine efficiency reduces actually and the acceleration power of the turbine becomes weak and the response ability of the acceleration is deteriorated.
When such a turbine is used as a gas turbine, the high temperature at the turbine inlet causes the increase of C0. In this case, a high temperature resistant material is required for the gas turbine. When the conventional material is used, the limitation of the strength of the material leads the restriction of the rotation velocity U of the blade 103, so that the theoretical velocity ratio U/C0 decreases. As a result, the turbine must be operated in the low efficiency point B.
To conquer such a technical problem, a mixed flow turbine is devised. FIGS. 4A to 4C show a conventional mixed flow turbine. In FIGS. 4A to 4C, the same or similar reference numerals are allocated to the same components as those of FIGS. 1A and 1B.
In the conventional mixed flow turbine, as shown in FIG. 4B, a gas inlet side edge of the blade 103′ is linear with a predetermined angle with respect to the direction of rotation. The blade attachment angle δ between an end point 106′ of a blade 103′ on the surface of the hub 102 on the gas inlet side and the line of the radial direction is set to a non-zero value, and is often set to 10-40°. In the case of the radial turbine, the blade attachment angle δ is set to zero. In the mixed flow turbine, the sectional profile of the blade 103′ taken out along the line I—I shown in FIG. 4B has a curved (parabolic) shape as a whole, including the neighborhood of the gas inlet, as shown in FIG. 4A.
The flow problem in a typical mixed flow turbine at the point B under the condition that the theoretical velocity ratio U/C0 decreases will be described below. FIG. 5 shows a relation between a blade angle βk and a flow angle β. Referring to FIG. 5, the flow angle β107 is about 20° and constant at the point B in the radial turbine. The blade angle βk108 of the radial turbine is zero and constant. In this example, the incidence i2 is about 20° and the efficiency decreases due to this incidence i2, compared with the maximum efficiency. On the other hand, in the mixed flow turbine, the flow angle β109 is about 20° on the side of the shroud but increases to about 40° on the side of the hub. Such a distribution of the flow angle β109 is caused by the characteristic of the mixed flow turbine because a rotation radius R106 is smaller than a rotation radius R111, as shown in FIG. 4C. As shown in FIG. 4C, R106 is the rotation radius at the distance between the end point 106′ of the blade 103′ on the hub side on an inlet side blade edge line and the rotation axis L. Also, the rotation radius R111 is the rotation radius at the distance between the end point 111′ of the blade 103′ on the shroud side on the inlet side blade edge line and the rotation axis L. When the rotation radius R106 becomes smaller than the rotation radius R111, as shown in FIG. 6, the rotation velocity U decreases. On the other hand, the circumferential component of the absolute flow velocity C increases inversely proportional to the radius by conservation of angular momentum, so that the flow angle β109 increases to about 40° on the hub side, as shown in FIG. 5. In this way, in the conventional mixed flow turbine, the incidence I2106 can be decreased on the side of the hub surface. To measure the increase of the incidence caused by the increase of the flow angle, the blade angle βk110 in the mixed flow turbine is set to about 40° on the hub side to approximately coincide with the flow angle. At this time, the incidence is shown by i2113.
In this way, the mixed flow turbine can be designed for the flow angle β and the blade angle βk to be near to each other on the hub side, and the incidence i2106 in the hub side can be made to be near to zero. The mixed flow turbine has such advantages. However, the flow angle β109 decreases linearly from the hub side to the shroud side, the blade angle βk110 decreases parabolically from the hub side and the shroud side. Therefore, the incidence i2112 is increased to a maximum value in a middle point 112 of the gas inlet side blade edge line. The losses in the mixed flow turbine increase due to the difference between the distribution of the flow angle and the distribution of the blade angle and the efficiency of the mixed flow turbine is reduced due to the increase of the incidence.
Therefore, a technique to increase the efficiency of a mixed flow turbine operated at a low theoretical velocity ratio U/C0 is needed.